fix(api): disallow some more functions during textlock

Problem: nvim_buf_set_text(), nvim_open_term() and termopen() all change buffer
text, which is forbidden during textlock. Additionally, nvim_open_term() and
termopen() may be used to convert the cmdwin buffer into a terminal buffer,
which is weird.

Solution: Allow nvim_buf_set_text() and nvim_open_term() in the cmdwin, but
disallow nvim_open_term() from converting the cmdwin buffer into a terminal
buffer. termopen() is not allowed in the cmdwin (as it always operates on
curbuf), so just check text_locked().

Also happens to improve the error in #21055: nvim_buf_set_text() was callable
during textlock, but happened to check textlock indirectly via u_save();
however, this caused the error to be overwritten by an unhelpful "Failed to
save undo information" message when msg_list == NULL (e.g: an `<expr>` mapping
invoked outside of do_cmdline()).

src/nvim/api/buffer.c

@@ -515,6 +515,7 @@ end:
 void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, Integer start_col,
                        Integer end_row, Integer end_col, ArrayOf(String) replacement, Error *err)
   FUNC_API_SINCE(7)
+  FUNC_API_TEXTLOCK_ALLOW_CMDWIN
 {
   MAXSIZE_TEMP_ARRAY(scratch, 1);
   if (replacement.size == 0) {